Species: Allonychia jamesi (Meek, 1872)
- 1872 Megambonia jamesi Meek, Proc. Acad. Nat. Sci. Philadelphia, p. 321.
- 1873 Megambonia jamesi Meek, Geol. Surv. Ohio, Pal. 1, p. 136, pl. 12, figs. 9a-b.
- 1873 Megambonia jamesi Miller, Cincinnati Quart. Jour. Sci., 1, p. 13.
- 1889 Ambonychia jamesi Miller, N.A. Geol. Pal., p. 460.
- 1893 Allonychia jamesi Ulrich, Geol. Surv. Ohio, 7, p. 641, pl. 48, fig. 7.
- 1908 Allonychia jamesi Cumings, 32d Ann. Rep. Dep. Geol. Nat. Res. Indiana, p. 986, pl. 43, fig. 2.
- 1909 Allonychia jamesi Grabau and Shimer, N.A. Index Fossils, 1, p. 432.
Map point data provided by iDigBio.
- C3 Sequence (Corryville)
Identification in Hand Sample:
- Large, convex shell
- Hinge is about two-thirds of the diameter of the valves
- Simple and depressed raditating rib structure
- Pointy umbo
- Prominent beaks rising above the hinge line
Allonychia jamesi from the McMillan Formation of Crestview Hills, Kentucky (OUIP 585)
- Shell attaining a rather large size, a little obliquely subovate in general form, rather convex, the most gibbous ‘part being somewhat above and in front of the middle, more or less abruptly cuneoate posteriorly and below; basal outline regularly rounded; posterior margin rounding into the base, and ascending with a convex curve and forward inclination to the posterior extremity of the hinge, which is not in the slightest degree alate; anterior margin rounding into the base below, and slightly sinuous under the lobe-like protuberance, or rudimentary wing above, which is convex. slightly more prominent than tile margin below, and defined from the swell of the umbonal regions on each side, by an oblique, sulcus extending to the hinge margin in front of each beak ; hinge equaling about two-thirds of the antero-posterior diameter of the valves; beaks rather prominent, or rising distinctly above the hinge line, but slightly oblique, ‘and distinctly incurved; umbonal slopes broadly rounded; longer axis of the valves moderately oblique to the hinge line. Surface ornamented by very regular, simple, and depressed radiating costae, a little wider than the furrows between, and numbering about five in a space of 0.30 inch. Near the middle of the lower margin.
- Height, about 2.05 inches; antero-posterior diameter, 2.16 inches; convexity, 1.50 inch.
- Two very nearly such groups actually exist in the Cincinnati rocks and I hope to publish descriptions of them in the next (7th) report of the state geologist of Ohio. One (Allonychia) will contain, besides the type, Ambonychia (Megambonia) jamesi Meek, two new species. They are all more erect shells, possessing a protruding byssal opening, a short hinge with wide ligamental area, but neither cardinal nor lateral teeth.
- The protrusion of the byssal opening, short, edentulous hinge, and non-terminal beaks are the characters that distinguish this genus from Byssonychia, The same features, excepting the one that relates to the absence of hinge teeth, also separate the genus from Anomalodenoia and Eridonychia. The presence of a large byssal opening and the short hinge sufficiently distinguish the new genus from Ambonychia as restricted by me. As to Megambonia, Hall, under which genus, because of an external resemblance, Meek and others have placed the typical species jamesi, it is enough to say that Allonychia is totally different internally. Indeed, the two genera cannot possibly belong to the same family.
- On plate 48, figure 7 illustrates a large and well-preserved cast of the interior of Allonychia jamesi. Comparing it with the exterior we learn that the test was thick on the anterior side both beneath and above the byssal protrusion; and that the umbo is more pointed and smaller in the cast thin of the shell, and not so much incurved. A small lobe is separated from the upper part of the byssal protrusion and thus lies immediately beneath the anterior extremity of the hinge. It is believed to be equivalent to a similar protuberance met with in internal casts of certain species of Byssonychia (e.g. B. intermedia, M. and W. sp.) and in Amphicoelia, Hall. Perhaps it is also to be likened to the subrostral lobe of Ambonychia. Though highly improbable it is still possible that the cavity of which it is the filling may have lodged an anterior adductor muscle. The feature should perhaps have been included in the generic diagnosis.